Check if path is child of directory

[Jeroen Bollen]

I'm building a webserver using the Vibe.d library. Whenever the 
user requests a page inside my /images/ folder; I want them to 
output this file.

Because there will be a lot of images present, and because these 
are likely to change in the future, I would like to just get the 
URL from the request, and automatically output the file.

I am aware though, that users could perform tricks like 
"images/../../../../sensitive_file_here". In order to prevent 
that I would like a solid way of making sure the entered path is 
actually inside the images directory.

How do I do this?